1. Field of the Invention
This invention relates to an apparatus for reproducing information. Also, this invention relates to a method of reproducing information. In addition, this invention relates to a computer program for reproducing information. Furthermore, this invention relates to an information recording medium.
2. Description of the Related Art
Optical recording discs include Digital Versatile Discs (DVDs) of various types. Information recorded on each DVD is divided into ECC blocks. Each ECC block has a plurality of frames whose heads are loaded with sync signals respectively. Portions of the frames which follow their heads are loaded with information pieces respectively. The frames have a fixed length, and the positions of the sync signals are spaced at equal intervals. Therefore, during the reproduction of the information from the DVD, the sync signals are reproduced at constant intervals. In the case where the reproduced sync signals are counted and the period of the occurrence of the reproduced sync signals is calculated from the count result, it is possible to predict, from the calculated sync-signal period, a time position at which a next sync signal will be reproduced.
A typical information reproducing apparatus includes a synchronization circuit for detecting sync signals from a reproduced signal and synchronizing the reproduction of information with the reproduced signal in response to the detected sync signals. It is known to provide a synchronization circuit with sync protection and sync insertion.
The sync protection predicts a time position of a next sync signal in a reproduced signal, and sets a window at the predicted time position. Once the synchronization with the reproduced signal is acquired, the sync protection is sensitive to only a sync signal in the window and is insensitive to that outside the window. Accordingly, the synchronization is protected against a wrong sync signal, that is, a sync signal outside the window.
The sync insertion is active once the synchronization with the reproduced signal is acquired. When a sync signal in the window fails to be detected, the sync insertion generates a pseudo sync signal usable as a detected sync signal. Accordingly, the synchronization is maintained even in the event that the detection of a sync signal fails.
In the case where a sync signal in the last frame in an ECC block and a sync signal in the first frame in a next ECC block are spaced at an interval different from the normal value, the prediction-based sync protection tends to be unreliable at and around the boundary between ECC blocks.
Japanese patent application publication number 2001-243727 discloses an apparatus for reproducing information from an optical disc. The information recorded on the optical disc is divided into a plurality of frames having a fixed length corresponding to a prescribed number of clock periods. The heads of the frames have sync signals, respectively.
The apparatus in Japanese application 2001-243727 includes a circuit for detecting sync signals in a reproduced bit stream. The time position of a next sync signal is predicted from the number of clock periods between the detected sync signals. The predicted time position is used as a center window. Every detected sync signal in the center window is used as normal one. First, second, third, and fourth off-center windows are set. The first off-center window precedes the center window by two bits. The second off-center window precedes the center window by one bit. The third off-center window follows the center window by one bit. The fourth off-center window follows the center window by two bits. For each of the first to fourth off-center windows, successively detected sync signals therein are counted. When the number of successively detected sync signals in one of the first to fourth off-center windows reaches a prescribed value, a corresponding sync corrective signal is generated. The sync corrective signal causes the center window to shift to a position equal to that of the off-center window in question. Therefore, the center window is updated and corrected. Then, detected sync signals in the correction-resultant center window are used as normal sync signals.